Manish Chandra, CEO of Poshmark, made a big bet on mobile when he launched his used-clothing e-commerce service. Poshmark launched with just an iPhone app, whereas most e-commerce plays start on the Web and then move to mobile.

But Chandra believed mobile is the future. And that bet has paid off pretty well — Poshmark is one of the hottest startups in fashion tech, moving over 1.5 million items each year. The Mountain View-based company is the second-best rated iPhone app in the fashion category, after Lithuania's Vinted, and it's raised about $16.6 million to date.

Here's what Chandra has to say about how mobile is a game changer for retail:

Tell me a bit about how you founded Poshmark. What was the opportunity you saw in fashion tech?

I’ve been in technology for about 20 years, and fashion technology for the past 8 or 9 years. I founded a company in 2005 called Kaboodle, which eventually was acquired by Hearst Media, and that was one of the first socially-focused fashion and women’s communities. It was kind of like Pinterest, but very much focused on fashion with a lot of lifestyle and home products thrown in. What I saw there were women trying to buy from each other items that they had bought, a kind of secondary marketplace that was forming, but obviously we weren’t built for that.

And then looking at my own closet, I saw that my wife had shopping bags full of stuff that she didn’t wear, that were never even used. And when you look at fashion as an industry, it’s so huge — we spend in the U.S. $250 billion on fashion, 80 percent of that on women’s fashion. So women are putting roughly a trillion dollars of clothes into their closets every three years. But it doesn’t all get worn — a third of it gets worn regularly, a third of it gets worn once or twice, and a third of it is worn not at all.

That just creates a huge opportunity for secondary sales, a market that barely even exists. If you look at the existing secondary fashion market, it's maybe $5 billion to $7 billion. If you’re being generous, $10 billion at most. That’s just tiny compared to the amount of transactions that could occur. And if you look at the biggest player in fashion e-commerce, eBay, which sells about $4 billion in fashion, they do maybe $6 or $7 hundred million in secondary sales. Primarily they do $3+ billion in first sales.

So obviously there’s a big opportunity. But then the iPhone 4 came out in 2010, and suddenly there’s an answer to this business problem. Suddenly it’s really easy to take very beautiful photos of items to list for sale right from the phone. And with that great screen on the iPhone 4, now you can actually browse these great looking listings without having to go back to your computer.

So I figured that the technology was finally there to solve this problem I’d been thinking about for a while. And in 2011 I went out and raised a round of capital from Mayfield Fund, and the app came out in 2011.

Initially my pitch was “If women are willing to sell on the phone, then women should be able to buy on the phone.” So we only built an iPhone app, we didn’t have anything else, we didn’t have a website, and that turned out to be a good bet.

We grew significantly in 2012, grew another 13 fold last year. Last year we moved over a million and a half items through the app, and now it looks like this space is fully formed. You’re starting to see many companies jump into it.

But here’s our metrics, which are crazy — every day women upload a million dollars worth of merchandise onto Poshmark, which means every couple of weeks we add basically an entire Nordstrom’s store to the app. And our users spend roughly 25 minutes a day, they’re deeply engaged. We just released a closet-sharing industry report, looking at what brands are moving. It really ranges, from Loius Vuitton and Chanel to more independent brands like Rag and Bone, and then also stuff that’s more mainstream like Coach. But it all sells. Things like Lululemon or Loius Vuitton move very quickly. But we also found a different cadence for shopping — Friday night is the big time for shopping, which is interesting because usually you see shopping on Saturday and Sunday. It’s evidence that mobile changes people’s shopping habits significantly.

What we’re seeing is mobile in particular as a technology having massive impact because it blends online and offline.

That’s interesting. Most of the existing players in fashion tech, in e-commerce really, they started on the Web and now they’re branching out into mobile. But you were mobile first from day one. What is the thinking behind that focus?

The reason, initially, was that it was just too hard to get people to do listings the old fashioned way — you had to get them to get out their camera, take a picture, take out the SD card, upload it to the web, do the listing. It was too much. But with a phone it’s really easy — take a couple of photos and you’ve merchandised your item. So that part’s really intuitive.

But a lot of the other thinking was, why do you want to build a store on the phone? The screen is small, it’s hard for her to see things, etc. My thinking was the phone is with her all the time.

And before I started the company, being a fairly old guy, I wanted to see if it was even possible. Like, this was 2008, the phone was an important tool but not nearly as important as it is today. So I decided for 3 months I was not going to use my laptop at all. Just my iPhone and iPad. And it was actually possible — today that doesn’t seem like a radical idea, particularly because iPhone has gotten so powerful.

Essentially people are spending all their time with their phone. So I realized we had to focus — building things on the phone is so different from the Web, there are a lot of things that are really hard, some things are easier, it’s just different. So I decided we’re going to bet on the phone. We’re either going to fail or we’ll succeed, but we will live and die by the iPhone.

How did that affect your business strategy?

Here’s a few things mobile has done, as well as challenges. Actually building for the phone is very challenging, but most of all its just different.

One thing we did was we eliminated the shopping cart, which was a big, bold decision. There’s no shopping cart in Poshmark, you either buy or you don’t. That cuts down the checkout time, it ups the conversion rate by a lot, but obviously it’s a very different experience. One of our design principals is, any time you make someone click on something get out a hammer and hit yourself over the head with it, because that’s how much pain you’re causing the customer.

But one very good thing that does happen is you can connect to the consumer 24/7. So when we were thinking about the possibilities of that, one of the things we thought about is how do you get women to engage regularly with the app? And the answer is we built a lot of social functionality into the app, built a platform where people can connect about fashion but also talk about other things, about their lives.

And then we overlayed that with the concept of a “Posh Party,” a virtual party that happens over the phone. They happen on the phone, but also we sometimes hold them in real life, in hotels and businesses across the country. And we organize them around themes. A theme could be Chanel or Lululemon or something, or a them could be around a type of product. Like a chunky jewelry theme, or a high-end handbag theme. Women participate in real time — we get tens of thousands of women joining these things, buying and selling with each other in real time. They get thousands of items listed, and then we provide tools that let you search within a posh party and find items that you like.

You just can’t make something like that work on a website. Because to do it on the Web, you have to be at your computer, on the website, physically going to the party. But with mobile it’s like, I could be sitting having coffee with my girlfriends, checking Poshmark every few minutes, seeing if there’s anything cool, and then buy or sell and go back to my coffee. It creates this deep engagement, and engagement that you carry with you.

And here’s the crazy thing — when you’re at a physical Poshparty, you have a glass of wine and you’re walking around looking at items, you could still be online participating in another one.

That’s the promise of mobile. It fairly fundamentally changes the game for fashion.

That doesn’t sound like a model that’s necessarily limited to fashion

I don’t think it’s limited to fashion. My thesis is, retail is going to a place where you can find products anywhere, transact anywhere, and then fulfill anywhere. And if you look at those three principals, it completely revolutionizes retail. Now you have a customer relationship that starts in a woman’s bedroom, follows her into the bathroom, goes with her to the store, leaves the store, follows her home, is with her when she goes to sleep at night. It’s a transcendent relationship, and it's 24 by 7. The winners will be the companies that can create that relationship with a consumer.

So, a concrete example of that is, you wake up and open the app of a national retail brand, say Macy’s, on your phone, and you like a yellow dress, think hey, I got to try it on. So you decide on your coffee break you’re going to go to the local Macy’s store. You do that, you find the dress, but for some reason you want it in pink and they don’t have it in pink. Easy. You take out your phone, you buy it from the store, it ships to your house and you’re done.

The best example of that today is Apple. Apple is doing a thing where you can shop on your phone, you can shop in the store, you can shop in the store without touching a retail clerk by just scanning what you want, and if you don’t have the inventory you can buy it in the store with your phone and it shows up at your house.

Mobile is a transcendent platform, and for fashion that’s really powerful because inventory interaction, inventory management is a huge asset. Apple has a few SCUs, but fashion has many SCUS so this tool becomes very powerful in management.

One of the things we’re thinking about is working with local stores, and how can we work with local stores to make their products more mobile. We think there’s a huge opportunity there. You can imagine, a local boutique integrated into Poshmark, integrated back into the boutique, that allows that inventory to be transcendent. A girl in New York could shop inventory in Haight-Ashberry, a girl in Haight-Ashberry could shop New York/SOHO, all through Poshmark, or you could shop in the store. That’s major.

So you think the future of all retail is mobile, that the transaction can become much more seamlessly integrated into a consumer’s life, that it can happen in the instant they decide to buy.

Yes. The future is mobile, and also wearables, which is a developing area of mobile. It’s really creating an alter-ego of the person virtually. We live in a physical world, but because of mobile we also now move in a virtual world, and that’s creating new opportunities and new challenges for every industry.